Eggshell consists of about 95 % calciumcarbonate. Thus in an alkaline environment it is a good source of carbonate.
Be aware that eggshells from either egg- breaking plants or hatcheries will always contain a substantial amount of protein. During storage of the raw untreated material this will allow considerable bacterial growth. This may result in (serious) health hazards and odor problems, if no precautions are taken.